Examples of gelling agents include, but are not limited to, clays such as Laponite???, Montmorillonite, Cloisite???; metal alkoxides such as those represented by the formula R???Si(OR)n and M(OR)n (where n can be 3 or 4, and M can be Ti, Zr, Al, B, or other metals); and colloidal particles based on metal hydroxides or oxides.